The store checkout page only allows for 1 gift redemption per transaction. If you would like to combine gift cards or adjust a gift card balance, start in the "Gifts" section of the portal.
Select the appropriate check box(es) on the left side of the screen for the recipient and choose the "Adjust balance" button on the pop-up banner at the bottom of the screen
Enter the new balance for the gift card and "Adjust balance".
You'll then see the "Remaining Amount" column reflects the gift amount changes.